1. Field of the Invention
The present invention relates to a resin sealed semiconductor device (or resin encapsulated semiconductor device) in which a plurality of semiconductor switching devices are stacked together and integrally sealed using a resin to reduce the footprint of the device, and also relates to a method for manufacturing such a resin sealed semiconductor device.
2. Background Art
IGBTs (Insulated Gate Bipolar Transistors) and power MOSFETs are semiconductor switching devices widely used for power conversion and motor control. There will now be described, with reference to FIG. 25, a three-phase AC inverter circuit using IGBTs which is an example of a circuit device employing semiconductor switching devices.
As shown in FIG. 25, this three-phase AC inverter circuit, 301, includes three inverter legs: a U-phase leg 302, a V-phase leg 304, and a W-phase leg 306. Each of the U-phase, V-phase, and W-phase legs 302, 304, and 306 is made up of an upper arm 310 and a lower arm 312. The upper arm 310 includes an IGBT 314 and a freewheel diode 316 connected in parallel to the IGBT 314, while the lower arm 312 includes an IGBT 318 and a freewheel diode 320 connected in parallel to the IGBT 318. The legs 302, 304, and 306 are connected to the same high voltage DC power supply and switched by their respective control signals sent from control circuitry so as to supply AC power components to a load 322.
It should be noted that it is common that such a circuit device containing a plurality of IGBTs (or semiconductor switching devices) is sealed, or encapsulated, with a resin. FIG. 26 shows an exemplary configuration of the U-phase leg 302 of the three-phase AC inverter circuit 301. Referring to FIG. 26, a first conductor electrode 334 and a second conductor electrode 332 are disposed on an insulating substrate 335.
The IGBT 314 and the freewheel diode 316 in the upper arm described above are disposed on the first conductor electrode 334 such that the collector of the IGBT 314 and the cathode of the freewheel diode 316 are in contact with the first conductor electrode 334. The IGBT 318 and the freewheel diode 320 in the lower arm described above, on the other hand, are disposed on the second conductor electrode 332 such that the collector of the IGBT 318 and the cathode of the freewheel diode 20 are in contact with the second conductor electrode 332. These components disposed on the first and second conductor electrodes 334 and 332 are interconnected by wires 308 thereby implementing the U-phase leg 302 shown in FIG. 25.
It should be noted that in FIG. 26 the IGBT 314 receives a control signal from a control terminal 338, and the IGBT 318 receives a control signal from a control terminal 340.
It is a common practice that an insulating substrate with devices mounted thereon is sealed using a resin. A general resin sealing process will be briefly described with reference to FIGS. 27 to 32. First, for example, semiconductor device assemblies (or inserts) 360 to be sealed with a resin are placed within the cavity, 357, formed by an upper die 350 and a lower die 352, as shown in FIG. 27. It should be noted that resin tablets 356 and a plunger 354 are set in place in a portion of the lower die 352 to supply a molding resin to the cavity.
Next, the upper and lower dies 350 and 352 are clamped together, as shown in FIG. 28. At that time, the molding resin (or resin tablets) to be delivered into the cavity is heated by the (heated) upper and lower dies, resulting in a reduction in the viscosity of the resin. The plunger 354 is then moved toward the inside of the cavity, thereby injecting the molding resin into the cavity (see FIG. 29). After that the temperatures of the upper and lower dies 350 and 352 are lowered to solidify the molding resin, 358, within the cavity (see FIG. 30). Lastly, the solidified molding resin (containing the semiconductor device assemblies) is retrieved from between the upper and lower dies 350 and 352, as shown in FIG. 31, and split in the desired manner, as shown in FIG. 32. This completes the resin sealing process.
This resin sealing method is useful when each semiconductor device assembly (or insert) to be sealed is manufactured to a thickness within a predetermined range. However, the method cannot be used to sufficiently improve the heat dissipation characteristics of the device, nor does it allow the footprint of the device to be reduced. Accordingly, there has been a need for an improved resin sealing technique.
The footprint of a resin sealed semiconductor device may be reduced by stacking its semiconductor elements (e.g., IGBTs) together in the thickness direction of the device. That is, in the case of the three-phase AC inverter described above, the IGBT of the upper arm of one of the U-phase, V-phase, and W-phase legs may be stacked on the IGBT of its lower arm, and this structure may be sealed with a resin. (See, e.g., Japanese Laid-Open Patent Publication Nos. 2006-049542, 2006-134990, 2004-193476, 2002-026251, 2004-047850, 2005-064116, 2005-064115, 2005-333008, and 2005-303018.) This allows the footprint of the leg to be approximately halved, as compared to when the semiconductor elements (IGBTs) are disposed side by side on the flat surface of the substrate, as shown in FIG. 26.
As described above, the density of components in the resin sealed semiconductor device can be increased by stacking each pair of IGBTs together, as compared to the case where a plurality of semiconductor elements (IGBTs) are disposed side by side on the flat surface of the substrate. In such a case, however, it is necessary to improve the heat dissipation characteristics of the resin sealed components (IGBTs, etc.), since power semiconductor devices need have good heat dissipation characteristics to maintain their performance. To achieve this, a semiconductor device assembly, or structure, 401 as shown in FIG. 34 may be formed and sealed with a resin. Specifically, the semiconductor device assembly 401 may be placed within a molding die and sealed with a resin. This assembly 401 includes an IGBT 410 and an IGBT 408 stacked on the IGBT 410 and also includes a diode 424 and a diode 426 stacked on the diode 424.
A heat spreader 422 of a metal is bonded to the emitter of the IGBT 408, for example, by solder 416 (see FIG. 34). (It should be noted that the emitter and gate of the IGBT 408 are formed on the top side of the IGBT.) The heat spreader 422 is also bonded to the anode of the diode 426. A copper foil 434 (a heat sink) is bonded through an insulating layer 430 to the surface of the heat spreader 422 opposite that to which the emitter of the IGBT 408 is bonded. The surface of the copper foil 434 opposite that in contact with the insulating layer 430 is exposed to the outside environment, even after the resin molding process.
As for the IGBT 410 disposed beneath the IGBT 408, a heat spreader 420 of a metal is bonded to the bottom surface of the IGBT 410. (It should be noted that the collector of the IGBT 410 is formed in this surface.) The heat spreader 420 is also bonded to the cathode of the diode 424. A copper foil 436 (a heat sink) is bonded through an insulating layer 432 to the surface of the heat spreader 420 opposite that to which the collector of the IGBT 410 is bonded. The surface of the copper foil 436 opposite that in contact with the insulating layer 432 is exposed to the outside environment, even after the resin molding process.
This semiconductor device assembly 401 (configured as described above) is sealed with a molding resin using a molding die made up of a lower die 402, an intermediate die 404, and an upper die 406 as shown in FIG. 33. The intermediate die 404 is placed on and in contact with an upper surface of the lower die 402, and the upper die 406 is placed on and in contact with the upper surface of the intermediate die 404 and another upper surface of the lower die 402 (see FIG. 33). These molding die members are clamped together (after the assembly 401 is placed within the cavity, 400, of the molding die). A resin sealing method using the lower, intermediate, and upper dies 402, 404, and 406 will now be described by comparing it with the conventional resin sealing method described above with reference to FIGS. 27 to 32.
It should be noted that since it is necessary to expose to the atmosphere the surfaces of the copper foils 434 and 436 of the semiconductor device assembly 401 opposite those in contact with the insulating layers 430 and 432, respectively, these surfaces must be pressed against inner walls of the cavity 400 when the assembly is placed in position within the cavity 400 and a molding resin is injected into the cavity. This prevents the molding resin from reaching these surfaces of the copper foils 434 and 436 when the semiconductor device assembly 401 is molded, since they are in close contact with inner walls of the cavity 400. In this way, portions of the copper foils 434 and 436 (i.e., the above surfaces in contact with inner walls of the cavity) can be exposed to the outside environment.
As a result, in operation of this sealed assembly 401, the heat generated therein is dissipated from the copper foils 434 and 436, as well as from the emitter terminal, 440, of the IGBT 408, the collector terminal, 438, of the IGBT 410, and the main electrode 428 shown in FIG. 34 (the emitter terminal 440 being connected to the heat spreader 422 and extending from the molding resin). Thus, this resin sealing method allows the manufacture of resin sealed semiconductor devices having good heat dissipation characteristics.
However, the above method is disadvantageous in that it requires that the thickness, B, of the semiconductor device assembly 401 be exactly equal to the depth of the cavity 400 (denoted by A in FIG. 33) of the molding die made up of the lower, intermediate, and upper dies 402, 404, and 406. With this arrangement, a molding resin can be injected into the cavity such that the resin does not reach portions (or surfaces) of the copper foils 434 and 436, thereby allowing these portions to be exposed to the outside environment even after this molding process. Furthermore, excessive force is not applied to the semiconductor device assembly 401 during the process (if the above requirement is met). However, the thickness B of the semiconductor device assembly 401 is apt to vary a certain amount when manufactured.
Therefore, it happens that the thickness B of the semiconductor device assembly 401 is smaller than the cavity depth. In such a case, the copper foil 434 is spaced from the facing inner wall of the upper die 406 by a gap 450, and the copper foil 436 is spaced from the facing inner wall of the lower die 402 by a gap 452, as shown in FIG. 35, even after the lower, intermediate, and upper dies 402, 404, and 406 are clamped together. If the semiconductor device assembly 401 is sealed with a molding resin 454 in this state, the resin flows to cover those surfaces of the copper foils 434 and 436 that must be exposed to the outside environment, due to the gaps 450 and 452, as shown in FIG. 36, making it difficult to improve the heat dissipation characteristics of the device.
It also happens that the thickness B of the semiconductor device assembly 401 is greater than the cavity depth A. In such a case, the lower, intermediate, and upper dies 402, 404, and 406 cannot be clamped closely together, as shown in FIG. 37. Specifically, for example, a gap 460 may be formed between an upper surface of the lower die 402 and the facing lower surface of the upper die 406 (see FIG. 37). Eliminating this gap requires clamping the lower, intermediate, and upper dies 402, 404, and 406 together with a very strong force. Such forcible clamping, however, may result in destruction of components (as indicated by reference numerals 462, 464, 466, and 468 in FIG. 38).
Thus there are known resin sealed semiconductor devices in which semiconductor switching devices such as IGBTs are stacked together in order to reduce the footprint. In the manufacture of such resin sealed semiconductor devices, each semiconductor device assembly including stacked semiconductor switching devices may be sealed with a molding resin using a molding die while a portion(s) of the assembly is maintained in close contact with inner walls of the molding die, which leads to improved heat dissipation characteristics of the assembly (or resin sealed semiconductor device). However, this sealing method is disadvantageous in that, due to the variation in the thickness of the semiconductor device assembly when manufactured, the molding resin may flow to entirely cover the semiconductor device assembly during the molding process, or the assembly may be broken or damaged when the die members are clamped together.